Device for laterally steering streamer cables

ABSTRACT

A device and a method for laterally steering a streamer cable towed underwater behind a seismic survey vessel. The device includes a cable-steering assembly rotatably attached to a streamer cable. The assembly includes a body to which one or more wings are mounted. The wings are arranged to pivot about pivot axes. The assembly is ballasted so that the pivot axes of the wings are largely in a vertical plane. A conventional cable-leveling bird is converted to one version of a cable-steering device by ballasting to maintain the pivot axes of the bird&#39;s wings largely vertical. With an orientation sensor for sensing the orientation of the wings, the cable-steering device adjusts the angle of the wings to provide a sideward component of force to steer the streamer.

BACKGROUND OF INVENTION

The invention relates generally to marine seismic prospecting and, morespecifically, to devices and methods for controlling the lateralposition of streamer cables towed underwater behind survey vessels.

In marine seismic exploration, instrumented cables, known as streamers,are towed underwater by a survey vessel. The streamers are outfittedwith a variety of electronic devices, including hydrophones that detectseismic signals transmitted into the water and reflected off geologicstructures beneath the sea floor.

Devices known as cable-leveling birds are attached to a streamer atintervals along its length to control the streamer's depth. The birdsare equipped with adjustable diving planes, generally referred to aswings, each having a pitch axis about which the wings can be pivoted bya motor to generate the lift needed to maintain the cable at a desireddepth. Most commonly, the birds are rotatably attached to the streamerand weighted to hang pendulously from the cable with the pitch axis ofthe wings below the cable. These birds are effective depth-controldevices.

It is not uncommon for a survey vessel to tow six or eight or morestreamers of lengths up to 12 km. Because the costs of lost survey timeand of replacing a damaged or lost streamer are so high, it is importantthat the streamers not become entangled during their deployment.Entanglement is more likely to occur in the presence of strong crosscurrents or while the survey vessel is turning to make another passacross the survey zone. To help avoid entanglement in turns, forexample, each streamer is often operated at a different depth. Whilethis technique provides some measure of entanglement control, it alsosubjects the cables to potentially strong shear layers of current thatvary considerably with depth, possibly increasing the risk ofentanglement. Generally the most satisfactory way to avoid entanglementwith conventional systems is to steer the vessel through wide turns andto overspace the streamers from each other. But these techniquesincrease cost and reduce the precision of the seismic image.

Paravanes and other devices are used to separate the streamers at theirhead ends close to the survey vessel. But lateral streamer control andstreamer position predictability diminish as cable tension lessens downthe lengths of streamers. The wake created by the seismic vessel createsa phenomenon known as “trouser” effect on the array. The streamers fanout port and starboard, creating a large void in the seismic coveragedirectly aft of the vessel. The streamers assume the shape of trousers.These voids must be resurveyed on subsequent passes known as “in-fill.”In-fill can increase the cost of seismic surveying by up to 20%. Lack ofrepeatability in processes and positional inaccuracies can reduce thequality of the seismic data and increase the cost by necessitatingin-fill. Thus, there is a need to provide a technique for lateralstreamer positioning to reduce the cost of operation and to improve thequality of the resultant seismic image.

Today's state-of-the-art seismic vessels have the capacity to deploy,tow, and recover up to 18 streamers. Existing deployment schemes limitthe degree to which streamers can be simultaneously deployed, whichgreatly increases the cost of operation. There is a need to provide forlateral streamer control during the deployment and recovery phases tosupport simultaneous streamer operation without entanglement.

SUMMARY OF INVENTION

Accordingly, a method embodying features of the invention is providedfor laterally steering a streamer. A bird assembly of the kind typicallyoperated in a depth-controlling orientation in which one or more wingsare pivotable about one or more pivot axes that lie generally in ahorizontal plane is operated in another way. The bird assembly isoperated in an orientation in which the one or more pivot axes of theone or more wings lie largely in a vertical plane to steer the streamerlaterally.

In another aspect of the invention, a method for converting acable-leveling bird into a cable-steering bird comprises ballasting thebird so that it operates underwater in an orientation in which each of apair of wings of the bird pivots about a pivot axis that lies largely ina vertical plane to steer the attached underwater cable laterally.

In yet another aspect of the invention, an underwater cable-steeringdevice comprises a connector attachable around the periphery of anunderwater cable section and rotatable about the cable. A control deviceincludes a body connected to the connector external of the cable. Thecontrol device includes a first wing at one side of the body and asecond wing at an opposite side of the body. The first wing pivots abouta first axis, and the second wing pivots about a second axis. The twoaxes may coincide, but do not intersect the cable. The control devicealso includes means for pivoting the wings about their pivot axes.Ballast means is provided to help ballast the steering device tomaintain the pivot axes largely vertical.

In another aspect of the invention, an underwater cable-steering devicecomprises a connector attachable around the periphery of an underwatercable section and rotatable about the cable. A control device includes abody connected to the connector external of the cable. The controldevice includes a shaft extending through the body and defining a pivotaxis. A first wing portion is connected to one end of the shaft at oneside of the body, and a second wing portion is connected to the otherend of the shaft at an opposite side of the body. The two wing portionsmay be unitarily formed as a single wing. The pivot axis does notintersect the cable. Ballast means is provided to help ballast thesteering device to maintain the pivot axis largely vertical.

In still another aspect of the invention, an underwater cable-steeringdevice comprises a connector attachable around the periphery of anunderwater cable section and rotatable about the cable. A control deviceincludes a body connected to the connector external of the cable. Afirst shaft extends from a first side of the body. A first wing attachedat an end of the first shaft can pivot about a first axis defined by theaxial rotation of the first shaft. The first axis does not intersect thecable. The control device also includes means for pivoting the wingsabout their pivot axes. Ballast means is provided to help ballast thesteering device to maintain the first axis largely vertical.

BRIEF DESCRIPTION OF DRAWINGS

These features and aspects of the invention, as well as its advantages,are better understood by reference to the following description,appended claims, and accompanying drawings, in which:

FIG. 1 is an isometric view, partly cutaway, of a cable-steering deviceembodying features of the invention on a section of streamer cable;

FIG. 2 is a top view of the cable-steering device of FIG. 1;

FIG. 3 is a cross section of the cable-steering device of FIG. 1 takenalong lines 3—3;

FIGS. 4A–4C are schematics representing various ways of ballasting acable-steering device as in FIG. 1;

FIG. 5 is a front elevation view of a cable-steering device as in FIG. 1with buoyant tank ballast;

FIG. 6 is a front elevation view as in FIG. 4, but with an aileroncontrol;

FIG. 7 is a front elevation view, partly in cross section, of anotherversion of cable-steering device embodying features of the inventionincluding wing tip tanks;

FIG. 8 is a schematic block diagram of the cable-steering devices ofFIG. 1 and FIG. 7;

FIG. 9 is an isometric view of a dual cable-steering arrangement usingtwo cable-steering devices as in FIG. 1; and

FIG. 10 is an isometric view of another version of cable-steering deviceembodying features of the invention including a single wing.

DETAILED DESCRIPTION

A device, embodying features of the invention, for laterally steering astreamer cable is shown in FIGS. 1–3. The cable-steering assemblyincludes two connectors or cuffs 20 rotatably attached to collars 22, 23affixed about the periphery of a streamer 24. Races are formed on thecollars to receive the connectors and allow them to rotate freely aboutthe streamer. An oversized stop 25 at the rear of the rear collar 23keeps the cuffs in position as the cable is towed in the towingdirection 27. Instead of rotating about collars encircling the streamer,the connectors could rotate about insert sections placed in-line betweentwo streamer sections. The insert sections would themselves rotatablyreceive the connectors. The cuffs shown in FIGS. 1–3 could be realizedas Quick Cuff™ connectors manufactured and sold by Input/Output, Inc. ofStafford, Tex., USA, and described in detail in U.S. Pat. No. 6,263,823,which is incorporated by reference. Alternatively, the connectors couldbe Quick Latch™ connectors, also manufactured and sold by Input/Output,Inc., and described in U.S. Pat. No. 5,507,243, incorporated byreference.

A streamer control device 26 has front and rear pylons 28, 29 thatinclude latching hardware to releasably connect the control device tothe connectors. The pylons extend from a main body 30, in the form of ahollow tube that houses electronic communication and control circuits31, a battery 37, and a drive mechanism 38, including a motor. Wings 32,33 extend from opposite sides of a wing support section 34 of the modulebody between the two pylons. Each wing is mounted on opposite ends 35′,35″ of a single shaft or on the ends of separate shafts. A drivemechanism inside the body rotates the single shaft (or the separateshafts) to pivot each wing about pivot axes 36, 37 defined by theshafts, which are offset from the cable and do not intersect its longaxis.

Thus far, the description of the cable-steering device essentially isthe same as that for a cable-leveling bird, such as the DIGICOURSE® 5010DIGIBIRD™ brand manufactured and sold by Input/Output, Inc. The purposeof the cable-leveling bird is to work in conjunction with other suchbirds attached along a streamer. To maintain the streamer at a desireddepth, pivot axes of the wings remain generally in a horizontal plane.In this way, the bird pivots the wings in pitch about the pivot axis. Asthe pitch angle of the wings changes, lift is adjusted and,consequently, the depth of the cable can be controlled. The weightdistribution and specific gravity of the cable-leveling bird/connectorsystem is such that it remains suspended beneath the cable with thepivot axes of its wings generally in a horizontal plane.

The cable-steering bird of the invention, however, is ballasted so thatthe pivot axes of its wings remain largely vertical (V), as shown inFIGS. 1–3. In this way, changing the angle α (of the wings steers thecable horizontally (H). To maintain the pivot axes 36, 37 generallyvertical, the cable-steering bird 26, the connectors 20, and anythingattached to them to rotate about the cable with them is ballasted tokeep specific gravity about the same as that of the cable itself. Oneway to achieve this is to make one of the wings 33 heavier than theother 32. This can be done, for example, by making the lower wing out ofa denser material or installing a weight 40, such as a lead or tungstenweight, in a void 42 within the wing. (The wings of cable-leveling birdsare typically solid and molded out of polyurethane.) The interior of oneor both wings can be hollow with a void that is empty or filled with afoam material 44, such as glass-sphere-filled polyurethane orglass-sphere-filled epoxy, to keep them lightweight without affectingtheir designed shape. Properly ballasted in this way, the bird iscapable of steering the cable to which it is attached. Even as the cable24 rotates within the connector 20 under tow, as shown in FIG. 3, thepivot axes 36, 37 of the wings remain largely in a vertical plane. Evenif the pivot axes of the wings are not perfectly vertical, as long assome component of the axes lies in a vertical plane, some amount ofsteering is possible. For example, if the cable-steering assembly isinsufficiently balanced and the pivot axes are at an angle of 45°relative to both horizontal and vertical, the wings can still provide ahorizontal component of force to steer the streamer.

As illustrated in FIGS. 4A–4C, some ways to properly ballast the cablesteering device include: a) making the upper wing light, the lower wingheavy, e.g., weighted at the lower end, and leaving the bird body as is(FIG. 4A); b) making the upper wing light, the lower wing light, and thebird body weighted enough to achieve neutral buoyancy (FIG. 4B); and c)making the upper wing heavy at the wing tip, the lower wing heavy at thewing tip, and the bird body light (FIG. 4C). Thus, the ballast may beapportioned among the wings and the bird body in a variety of ways tomaintain the wings generally in a vertical plane.

Other ways of maintaining the pivot axes 36, 37 of the wings 32, 33largely vertical are shown in FIGS. 5 and 6. In FIG. 5, a buoyant tank46, or float, is attached to the bird body 26 as an appendage. The tanklowers the specific gravity of the bird assembly. Adjusting the volumeof the tank or the length of its connecting arm 48 adjusts the specificgravity of the cable-steering assembly to maintain the pivot axesvertical. Adding flotation in this way can be used alone or inconjunction with adjusting the absolute and relative weights of thewings. Both these means for ballasting are effective in properlyorienting the wings. Optionally, a weight 50, negatively buoyant, can beattached to the connector at a position on the opposite side of thecable to right the pivot axes of the bird assembly. These ballastingmeans may be used to preadjust the cable-steering assembly beforedeployment underwater. They are also hydrostatic in that they do notdepend on the speed of the tow to be effective.

Another way to maintain the pivot axis of the wings vertical is shown inFIG. 6. In this version, a rudder or an aileron 52 is controlled by anaileron controller 54 attached to the connector 20 on the opposite sideof the streamer from the cable-steering device 26. Alternatively, theaileron 52′ could extend from the cable-steering device directly. Theaileron rotates about a generally horizontal axis 56 similar to thewings of a cable-leveling bird and provides more or less lift to thecable-steering assembly as a function of its pitch angle of attack. But,in this version, the amount of lift depends on the speed of the streamerthrough the water. The aileron controller may include an orientationsensor to determine its orientation relative to vertical.

Another version of cable-steering device is shown in FIG. 7. In thisversion, the wings 90, 91 are tipped with bulbous portions, or tanks 92,93, which provide more volume for ballast control. In the example, theupper wing includes a greater volume of low-density material than theupper wing of FIG. 1. The lower end of the lower wing may include adenser molded or fill material 94 or a weight 95 in the lower wing-tiptank.

The wing control portion of the cable-steering bird is shown in FIG. 8.A controller 59, preferably including a microprocessor, receives signals60, 61 representing the orientation of the bird body relative tovertical as defined by the gravity vector. Orientation sensors, such asan inclinometer 62 or an accelerometer 63, are used to determine theorientation. In some cases, an inclinometer alone may be sufficient. Inother cases, in which cable accelerations are frequent and significant,multiple-axis accelerometers may be necessary. From the orientationsensor signals, the controller can determine the orientation of thewings. The cable is steered by adjusting the angle of attack of thewings 32, 33. A shipboard controller keeping track of all the streamersbeing towed determines what action each cable-steering device shouldtake. The shipboard controller communicates that action to thecontroller in the cable-steering device, which adjusts the wingsaccordingly. A signal 64 representing a change in the wing angles anddetermined by the controller from its computation of the orientation ofthe device and from the steering command is sent to the wing drivemechanism 38, which includes one or more wing actuators 64. The wingactuators rotate the shaft or shafts 35, changing the angles of attackof the wings to change and, consequently, the lateral force on thestreamer. The wings can be controlled independently by separateactuators and shafts or in unison by a single actuator and a singleshaft.

As shown in FIG. 9, it is also possible to attach a pair ofcable-steering devices 26, 26′ to a streamer 24 with a connector 20.Each device is connected to the connector circumferentially spaced 180°to be positioned on opposite sides of the streamer. This arrangementwould provide more wing surface area to exert greater lateral forces forsteering the streamer. Each cable-steering device is ballasted, forexample, by a weight in the lower wing, to maintain the wings in agenerally vertical plane.

A single-wing version of cable-steering bird embodying features of theinvention is shown in FIG. 10. While multi-wing versions makeindependent wing angle control for roll compensation possible, a singlewing version provides dedicated lateral steering. This version iscontrollable by a controller as in FIG. 8. As in FIG. 1, a main body 66attaches to a connector 20 that allows the body to be rotatable about astreamer cable. Like the body 30 in FIG. 1, the body 66 houseselectronic communication and control circuits, a battery, and a drivemechanism, including a motor. Opposite ends 68, 69 of a shaft 70 extendfrom opposite sides of the body. A wing 71 includes a first wing portion72 and an opposite second wing portion 73. The wing is formed of apolyurethane outer skin and an internal filler of glass-sphere-filledepoxy, for example, for lower density. Each wing portion is connected toan end of the shaft at attachment arms 74, 75. The shaft defines a pivotaxis 78 about which the wing pivots. The two wing portions arepreferably formed unitarily. The wing is ballasted so that the pivotaxis lies largely in a vertical plane for laterally steering a streamercable. The first wing portion, for example, can be ballasted with amaterial whose density is greater than that of water to urge it to ridebelow the streamer. The second wing portion, for example, can beballasted with one or more voids that may be filled with a material lessdense than water to urge it to ride above the streamer. With these andthe other ballasting techniques already described, the single wing canbe made to ride through the water with vertical stability.

Although the invention has been described with respect to a fewpreferred versions, other versions are possible. For example, the anglesof each wing could be changed relative to each other to help maintainthe wing pivot axes vertical. As another example, floats can be added toballast the cable-steering assembly at various positions around theperiphery of the connectors, to the body of the assembly at variouspositions, or internal to the body itself. So, as these few examplessuggest, the scope of the invention is not meant to be limited to thepreferred versions described in detail.

1. An underwater cable steering device comprising: a connector attachable around the periphery of an underwater cable and rotatable about the cable; a control device including: a body connected to the connector external of the cable; a first wing disposed at one side of the body and arranged to pivot about a first axis; a second wing disposed at an opposite second side of the body and ranged to pivot about a second axis; wherein the first axis and the second axis do not intersect the cable; means for pivoting the first wing about the first axis and the second wing about the second axis, wherein the means for pivoting comprises: a first shaft extending from the first side of the body and affixed to the first wing and defining the first axis; a second shaft extending from the second side of the body and affixed to the second wing and defining the second axis; and a drive mechanism connected to the first shaft and the second shaft to rotate the first shaft along the first axis and the second shaft along the second axis, thereby to adjust the angle of attack of the wings; ballast means to help ballast the control device to maintain the first axis and the second axis largely vertically disposed.
 2. A device as in claim 1 wherein the ballast means includes a weighted portion in the first wing.
 3. A device as in claim 1 wherein the ballast means includes a foamed portion in the second wing less dense than the rest of the second wing.
 4. A device as in claim 1 wherein the first and second wings each include hollowed portions.
 5. A device as in claim 1 wherein the first and second wings each include tanks at the wing tips.
 6. A device as in claim 1 wherein the first shaft and the second shaft are axially aligned.
 7. A device as in claim 1 wherein the first shaft and the second shaft are opposite ends of the same shaft.
 8. A device as in claim 1 wherein the drive mechanism includes a first actuator coupled to the first wing and a second actuator coupled to the second wing.
 9. A device as in claim 1 wherein the drive mechanism includes an actuator coupled to both wings.
 10. A device as in claim 1 wherein the control device includes an orientation sensor producing a signal representing the orientation of the body.
 11. A device as in claim 1 wherein the control device includes an accelerometer producing a signal representing the acceleration of the body through the water.
 12. A device as in claim 1 wherein the fast wing is heavier than the second wing.
 13. A device as in claim 1 wherein the ballast means comprises a negatively buoyant body attached to the connector at a circumferentially offset position from the control device around the periphery of the connector.
 14. A device as in claim 1 wherein the ballast means includes a positively buoyant appendage extending from the body.
 15. A device as in claim 1 wherein the ballast means includes a weight in the first wing.
 16. A device as in claim 1 wherein the ballast means includes a float attached to the connector.
 17. A device as in claim 1 further comprising a second control device connected to the connector on the opposite side of the underwater cable from the other control device.
 18. An underwater cable steering device comprising: a connector attachable around the periphery of an underwater cable and rotatable about the cable; a control device including: a body connected to the connector external of the cable; a shaft extending through the body and defining a pivot axis; a first wing portion connected to one end of the shaft at one side of the body to pivot about the pivot axis; a second wing portion connected to the opposite end of the shaft at an opposite side of the body to pivot about the pivot axis; a drive mechanism connected to the shaft to rotate the shaft about the pivot axis to adjust the angle of attack of the wing portions; wherein the pivot axis does not intersect the cable; ballast means to help ballast the control device to maintain the pivot axis largely vertically disposed.
 19. A device as in claim 18 further comprising a second control device connected to the connector ante opposite side of the underwater cable from the other control device.
 20. A device as in claim 18 wherein the ballast means includes a weighted portion in the first wing portion.
 21. A device as in claim 18 wherein the ballast means includes a foamed portion in the second wing portion less dense than the rest of the second wing portion.
 22. A device as in claim 18 wherein the first and second wing portions each include hollowed portions.
 23. A device as in claim 18 wherein the drive mechanism includes an actuator coupled to both wing portions.
 24. A device as in claim 18 wherein the control device includes an orientation sensor producing a signal representing the orientation of the body.
 25. A device as in claim 18 wherein the control device includes an accelerometer producing a signal representing the acceleration of the body through the water.
 26. A device as in claim 18 wherein the first wing portion is heavier than the second wing portion.
 27. A device as in claim 18 wherein the ballast means comprises a negatively buoyant body attached to the connector at a circumferentially offset position from the control device around the periphery of the connector.
 28. A device as in claim 18 wherein the ballast means includes a positively buoyant appendage extending from the body.
 29. A device as in claim 18 wherein the ballast means includes a weight in the first wing portion.
 30. A device as in claim 18 wherein the ballast means includes a float attached to the connector.
 31. A device as in claim 18 wherein the first wing portion and the second wing portion are unitarily formed.
 32. An underwater cable steering device comprising: a connector attachable around the periphery of an underwater cable and rotatable about the cable; a control device including: a body connected to the connector external of the cable; a first shaft extending from a first side of the body; a first wing attached to an end of the first shaft and arranged to pivot about a first axis defined by the axial rotation of the first shaft; a drive mechanism coupled to the first shaft to rotate the first shaft on the first axis to adjust the angle of attack of the first wing; wherein the first axis does not intersect the cable; ballast means to help ballast the control device to maintain the first axis largely vertically disposed.
 33. A device as in claim 32 further comprising a second control device connected to the connector on the opposite side of the underwater cable from the other control device.
 34. A device as in claim 32 wherein the ballast means includes a weighted portion in the first wing.
 35. A device as in claim 32 wherein the ballast means includes a foamed portion in the first wing less dense than the rest of the first wing.
 36. A device as in claim 32 wherein the ballast means includes a hollowed portion in the first wing.
 37. A device as in claim 32 wherein the first wing includes a tank at the wing tip.
 38. A device as in claim 32 wherein the control device further includes: a second wing attached to an end of the first shaft opposite the first wing and arranged to pivot about the first axis defined by the axial rotation of the first shaft.
 39. A device as in claim 38 wherein the first wing and the second wing are unitarily formed as a single wing structure.
 40. A device as in claim 32 wherein the control device further includes: a second shaft extending out of the second side of the body opposite the first shaft and defining a second axis not intersecting the cable; a second wing affixed to the second shaft; wherein the drive mechanism is coupled to the second shaft to rotate the second shaft along the second axis, thereby to adjust the angle of attack of the second wings.
 41. A device as in claim 40 wherein the first shaft and the second shaft are axially aligned.
 42. A device as in claim 40 wherein the first shaft and the second shaft are opposite ends of the same shaft.
 43. A device as in claim 40 wherein the drive mechanism includes a first actuator coupled to the first wing and a second actuator coupled to the second wing.
 44. A device as in claim 40 wherein the drive mechanism includes an actuator coupled to both wings.
 45. A device as in claim 32 wherein the control device includes an orientation sensor producing a signal representing the orientation of the body.
 46. A device as in claim 32 wherein the control device includes an accelerometer producing a signal representing the acceleration of the body through the water.
 47. A device as in claim 40 wherein the first wing is heavier than the second wing.
 48. A device as in claim 32 wherein the ballast means comprises a negatively buoyant body attached to the connector at a circumferentially offset position from the control device around the periphery of the connector.
 49. A device as in claim 32 wherein the ballast means includes a positively buoyant appendage extending from the body.
 50. A device as in claim 32 wherein the ballast means includes a float attached to the connector. 